The present invention generally relates to an image forming apparatus employing a photoreceptive sheet and an image receiving sheet and more particularly, to a method of controlling transport of the photoreceptive sheet to an image forming exposure position and an image forming position.
An image forming method employing an image forming sheet is disclosed in, for example, in Japanese Patent Laid-Open Publication No. 88739/1983 which employs not only a photoreceptive sheet coated with photosensitive resinous microcapsules containing colorless dye and photo-setting material but an image receiving sheet coated with developing material for effecting color development of the colorless dye. In this known image forming method, a selectively set image has been formed on the photoreceptive sheet by image forming exposure, the image receiving sheet is placed on the photoreceptive sheet so as to be pressed against the photoreceptive sheet such that unset photosensitive microcapsules are ruptured. Thus, the colorless dye contained in the photosensitive microcapsules is subjected to color development on the image receiving sheet to obtain an image.
Therefore, in an image forming apparatus based on such a known image forming method, the photoreceptive sheet is transported through an exposure position for performing image forming exposure and an image forming position for pressing the photoreceptive sheet, while the image receiving sheet is transported through the image forming position. Hence, the image receiving sheet is placed on the photoreceptive sheet having the selectively set image formed thereon by image forming exposure at the exposure position so as to be pressed against the photoreceptive sheet at the image forming position.
In order to obtain an excellent image on the image receiving sheet, the photosensitive microcapsules of an exposed portion of the photoreceptive sheet should be set sufficiently and complete reaction between the colorless dye and the developing material should take place on the image receiving sheet. The rate of photo-setting reaction of the photosensitive microcapsules is different from the rate of color development reaction of the colorless dye on the image receiving sheet. Therefore, in order to obtain a proper image, not only is feed rate of the photoreceptive sheet at the exposure position required to coincide with an optimum exposure speed for effecting photo-setting reaction of the photosensitive microcapsules but the feed rate of the photoreceptive sheet at the image forming position is required to coincide with an optimum image forming speed for effecting color development reaction of the colorless dye.
However, in the above described prior art image forming apparatus, a pressing device for pressing the photoreceptive sheet and the image receiving sheet against each other is constituted by a pair of rollers such that the photoreceptive sheet is transported through rotation of the rollers. Therefore, the transport speed of the photoreceptive sheet coincides with the peripheral speed of the rollers and thus, the feed rate of the photoreceptive sheet at the exposure position is identical with the feed rate of the photoreceptive sheet at the image forming position. Consequently, it becomes impossible to set either one or both feed rates of the photoreceptive sheet at the exposure position and the image forming position to the optimum speed or the optimum speeds, thereby resulting in deterioration of the formed image.